Automatic transmissions have a plurality of fluid operated friction devices, clutches and brakes, that control the drive ratio between the transmission input and the transmission output. Many of these friction devices are piston operated type mechanisms which include a disc pack having members (discs or plates) alternately connected between an input structure (a shaft, hub or gear) and an output structure (a shaft, hub or gear). In most instances, one set of discs has a friction material at the interface and another set of discs has a metal surface at the interface. The friction device is engaged to establish a drive ratio (1.sup.st gear, 2.sup.nd gear, etc.) between the transmission input and output.
The engagement of one friction device and the simultaneous disengagement of another device is generally termed a ratio interchange or shift. This interchange may be an upshift, increasing speed ratio or a downshift, decreasing speed ratio, and is often made under power transfer conditions during vehicle acceleration such that a large amount of heat energy is created in the friction device.
Current power transmissions disperse the heat energy with a large heat sink, generally a metal housing and metal discs, fluid cooling and a large number of friction discs or a large disc interface surface area. The friction device is sized to accommodate a number of successive high energy shifts which can occur during extended hill climbing or under abusive driving conditions. Under these design criteria, the friction device is significantly larger, either axially or radially or both, than is required for any one high energy shift since the powertrain must be protected from the cumulative temperature rise resulting from a series of high energy shifts. The energy transfer protection factor thus built into the powertrain serves to increase the size, weight and cost of the transmission, particularly when the system can employ at least two rotating clutches and two stationary clutches or brakes.